It took hackers less than a week to produce a working exploit that attacks a new, unpatched vulnerability in Microsoft's Internet Explorer, security firms said Tuesday.

Phel.a, a Trojan horse discovered Monday, attempts to exploit the flaw in Internet Explorer 6.0 dubbed "Microsoft Internet Explorer HTML Help Control Local Zone Security Restriction Bypass" that was first made public less than a week before, on December 21.

Symantec posted an alert on its Web site Monday, and rated the exploit as a "1," the lowest in its 1 through 5 ranking system.

The vulnerability in IE, which affects the version included with Windows XP SP2, the massive security update Microsoft rolled out in October, can result in a compromised, hacker-controlled machine if the user is drawn to the attacker's malicious Web site. The Trojan can be planted as part of a HTML page on the site.

If Phel.a successfully executes, it downloads additional code from another server (which as of mid-morning Tuesday, was offline), including a backdoor component that can be used by the attacker to later access the infected PC.

The flaws in IE behind this vulnerability are currently unpatched, although users can stop the Trojan either with anti-virus software or by disabling IE's Active scripting.